The present invention relates to a device for controlling a pulse motor, and more particularly to an improvement of its control when a driving switch is turned off.
A pulse motor, as one type of a brushless motor, is used in various fields such as in the field of air conditioning devices.
As illustrated in FIG. 2, a typical conventional device for controlling a pulse motor comprises a power source V.sub.B, a power source circuit 1, a control circuit 2, a driving circuit 3, and a driving switch SW. A power supply line V1 is connected to one end of the power source V.sub.B, and a ground line V0 is connected to the other end thereof. The driving switch SW is connected to the power supply line V1. The power source circuit 1 and the driving circuit 3 are connected to the power source V.sub.B via the driving switch SW.
The power source circuit 1 includes a constant voltage generating circuit 11, a reverse connection preventive diode D, and a noise absorbing electrolytic capacitor C. When the driving switch SW is in the ON-state, the constant voltage generating circuit 11 receives, via the driving switch SW and diode D, an electric power from the power source V.sub.B and supplies a constant voltage to the control circuit 2. One pole, i.e., positive pole, of the electrolytic capacitor C, is connected to between the constant voltage generating circuit 11 and the diode D, while the other pole, i.e., negative pole, is connected to the ground line V0.
The driving circuit 3 includes six (6) MOS type FET 31 to FET 36 (switching elements) for controlling the supply of electric current to driving coils L1, L2 and L3 of a stator of the pulse motor. Drains of the FET 31 to FET 33 are connected to the power supply line V1, and sources of the FET 34 to FET 36 are connected to the Ground line V0. A source of the FET 31 and a drain of the FET 34 are connected to each other, a source of the FET 32 and a drain of the FET 35 are connected to each other, and a source of the FET 33 and a drain of the FET 36 are connected to each other. Gates of the FET 31 to FET 36 receive high level control signals from output terminals of the control circuit 2, respectively and are turned on in response to the control signals thus received. The FET 31 to FET 36 respectively include parasitic diodes, i.e., built-in diodes D1 to D6, which are interposed between the sources and the drains. Cathodes of the built-in diodes D1, D2 and D3 are connected to the power supply line V1. Anodes of the built-in diodes D4, D5 and D6 are connected to the Ground line V0. One end of the driving coil L1 is connected to between the FET 31 and FET 34, while the other end thereof is connected to between the FET 32 and FET 35. One end of the driving coil L2 is connected to between the FET 32 and FET 35, while the other end is connected to between the FET 33 and FET 36. One end of the driving coil 3 is connected to between the FET 31 and 34, while the other end is connected to between the FET 33 and 36.
With the above-mentioned construction, when the driving switch SW is turned on, a constant voltage from the power source circuit 1 is supplied to the control circuit 2, thereby activating the control circuit 2. In response to a detection pulse coming from a sensor (not shown) for detecting the rotation of a rotor of the pulse motor, the control circuit 2 outputs a high level control signal so that a pair of FET are selectively turned On one by one. When the FET 31 and FET 35 are simultaneously turned on by outputting control signals A1 and A5, an electric current flows through the driving coil L1 in a direction as shown by an arrow in FIG. 2, thus generating a force for attracting the rotor. Similarly, when the FET 32 and FET 34 are simultaneously turned on by outputting control signals A2 and A4, an electric current flows through the driving coil L1 but in an opposite direction to the direction as shown by the arrow, thus generating a force for repulsing the rotor.
When the FET 32 and 36 are simultaneously turned on by outputting the control signals A2 and A6, an electric current flows through the driving coil L2 in a direction as shown by an arrow in FIG. 2. When the FET 33 and FET 35 are simultaneously turned on by outputting control signals A3 and A5, an electric current flows through the driving coil L2 but in an opposite direction to the direction as shown by the arrow.
When the FET 31 and 36 are simultaneously turned on by outputting the control signals A1 and A6, an electric current flows through the driving coil L3 in a direction as shown by an arrow in FIG. 2. When the FET 33 and FET 34 are simultaneously turned on by outputting control signals A3 and A4, an electric current flows through the driving coil L3 but in an opposite direction to the direction as shown by the arrow.
As described above, the FET pairs thus turned on have six (6) combinations. By selecting the combinations in a known order, a rotational magnetic field is generated in the stator of the pulse motor and the rotor of the pulse motor is rotated in a predetermined direction. The rotation of the pulse motor is stopped by turning off the driving switch SW.
The control device having the above-mentioned construction has the following drawbacks. Since no electric current is supplied to the driving coils L1 to L3 from the power source V.sub.B, the rotor is rotated merely by inertia. A braking current is supplied to the driving coils L1 to L3, and a magnetic field is generated in the driving coils L1 to L3. As a result, the rotor is abruptly stopped. A stopping sound (so-called "shaking noise") is generated at the time. This makes it impossible to fully enjoy one of the important advantages, i.e., low noise, of the pulse motor.
The present inventor has guessed or anticipated a possible cause of generation of a braking current as follows. Even if the driving switch SW is turned off, the control circuit 2 is kept operated until after the completion of discharge, by electric charge which has been accumulated within the electrolytic capacitor C in the power source circuit 1, and closed circuits including one of the driving coils L1 to L3 are formed one after another. For example, when the FET 32 is in the ON-state in FIG. 2, a closed circuit is formed by the FET 32 which is in the ON-state, the driving coil L2 and the built-in diode D3 of the FET 33. As a consequence, when a permanent magnet of the rotor is moved across the driving coil in the closed circuit, a braking current B1 is generated in the driving coil L2.